Modern integrated circuits ("ICs") are composed of up to several million individual devices, such as transistors and capacitors, fabricated on a single-crystal substrate of a semiconductor material, such as silicon. The processes used in the fabrication of ICs are continuously being improved to allow the manufacture of ever smaller devices to increase device speed and density in the IC. Faster and denser ICs lead to more powerful and less expensive electronic products, such as computers and video equipment.
Contamination control is a vital aspect of modern IC fabrication. Contamination, such as microscopic particles, on the wafer surface adversely affects the performance of electronic devices fabricated over the contamination. As smaller lines are used to compose and connect the devices, small particles that were previously undetected or ignored can interfere with the functioning of the circuit. Line widths of 0.5 .mu.m and smaller are currently being used in the fabrication of ICs, and contamination as small as 0.1 .mu.m is suspected to adversely affect device yield, i.e., the number of good die per wafer.
Contamination particles are typically detected using commercially available systems that scan the wafer surface with laser light and detect light reflected by the silicon surface and light scattered by particles on the surface. Most of the light is reflected by the highly polished wafer surface directly into a "light channel" photomultiplier or detector. Any particle in the path of the light beam will scatter some of the light into a "dark channel" photomultiplier. If a particle is sufficiently large, it will cause a significant decrease in the light intensity at the light channel photomultiplier. Signals from both the light and dark channel photomultipliers are processed and analyzed to detect the presence of particles.
ICs are typically fabricated in clean rooms or sealed environments having filtered air with extremely low airborne particle levels to reduce the number of particles deposited onto the wafers. Process liquids that contact the wafers are finely filtered to reduce the number of particles attracted to the wafer surface from process liquid. Prior to production of ICs, wafers are also thoroughly cleaned using highly filtered cleaning chemical solutions to remove particles on the surface. The entire processing environment, including the air, process gases and liquids, equipment, and equipment operators, are strictly controlled to improved process yield. To monitor the cleanliness of the IC processing environment, extremely clean wafers known as "monitor" wafers are used to detect the number of particles deposited on the production or "prime" wafers during various process steps. Although the electrical properties of monitor wafers are not critical, the surface of monitor wafers should be free of particles, pits, or other such light scattering anomalies that can be mistaken for particles deposited from the environment being monitored. Monitor wafers are, therefore, cleaned very thoroughly before use.
It has been found, however, that cleaning the polished silicon wafers does not reduce the number of light scattering anomalies below a certain level. In fact, repetitively cleaning polished silicon wafers using a standard SC1 cleaning process comprising NH.sub.4 OH, H.sub.2 O.sub.2, and H.sub.2 O actually increases the number of light scattering anomalies detected by laser inspection systems as reported by Ryuta, et al., "Crystal-Originated Singularities on Si Wafer Surface after SC1 Cleaning," 29 Japanese Journal of Applied Physics L1947-49. Cleaning, therefore, improves monitor wafer surfaces up to a point, but further cleaning actually appears to produce additional light-scattering anomalies. As the line width of devices becomes ever-smaller, improved monitor wafers are required to achieve to improve the cleanliness of the wafer processing environment for the electronics industry.